Ordered arrays of crystalline-Si (c-Si) microwires, fabricated by the chemical-vapor-deposition, vapor-liquid-solid (CVD-VLS) growth mechanism, were pioneered nearly five years ago for sunlight-to-electrical power conversion. P-type Si microwire arrays, employing a thin n+-doped emitter layer to form a buried junction (n+p-Si), have since realized sunlight-to-electrical power-conversion efficiencies>7% from solid-state photovoltaic (PV) devices, and >5% power-conversion efficiency toward H2 evolution from acidic aqueous electrolytes when functionalized with Pt electrocatalysts. In the absence of additional processing-intensive steps for light absorption enhancement, these devices demonstrated a short-circuit (maximum) current density (jsc)≈9 mA/cm2, open-circuit (maximum) photopotential (Voc)≈0.53 V, and fill factor≈70%. The product of these three terms determines the power-conversion efficiency of the device. The Si microwire geometry uses ˜5% of the material required for conventional wafer-based photovoltaics (PVs) and absorbs ˜20% of above bandgap sunlight. Various designs to alter the path of light and increase absorption by the Si microwire arrays, and thus jsc and the efficiency, have been investigated with modest success.
Recent photoelectrochemical study of radial junction Si microwire arrays exhibited 520 mV open-circuit voltages under AM 1.5 G 1-sun illumination, with the pH-independent, one-electron, outer sphere, methyl viologen redox system. Thus, the problem to-date is that with solely a single Si microwire array, increasing the efficiency beyond ˜7% is difficult.